- It's easier to find a place to shoot, say, an M&P 15-22 than it is a 5.56 M4gery when you live in the city.
- Ammo is a lot cheaper, making it less spendy to reinforce basic marksmanship skills.
- 500 rounds in a day in a .45 will leave your hands feeling like you've been hitting a heavy bag. 500 rounds in a plastic nine will still leave you knowing that you've been shooting when you pick up your utensils at dinner that night. 500 rounds of .22 will have you asking "Is that all we brought? Who wants to run to WalMart for more?"
(Plus, I think I've shot a couple of those very guns.)